HK40057673A - Optical waveguide tuning element - Google Patents

Description

Optical waveguide tuning element

Background

The optical waveguide device may be formed on a substrate such as silicon, with the waveguide cladding layer being formed of a first material (e.g., silicon oxide) and the core material being substantially square or rectangular in cross-section and being formed of a second material (e.g., silicon, Si, or silicon nitride, SiN, etc.) having a higher refractive index than the cladding material. Because light is substantially guided within the core material, the structure formed by the core material is often referred to as a waveguide, without involving a cladding material, which in some examples may be air or a dielectric with a lower index of refraction than the core material. The cross-sectional dimensions of the waveguide (i.e., the core material) may depend, at least in part, on the material used to form the waveguide, and in particular, on the refractive index of the waveguide material.

Tunable waveguide devices may be fabricated, for example, using movable elements (e.g., MEMS devices or larger optical elements), thermo-optic effects, or electro-optic effects. In the latter two examples, tuning is achieved by changing the optical properties (e.g., refractive index) of the waveguide (i.e., core) material by passing a current through, for example, a heating element positioned proximate to the waveguide. Inaccuracies in the movement of the movable element or changes in optical characteristics (e.g., due to lack of control over heating effects in devices using thermo-optic effects) result in tuning errors, and this can significantly impair the performance of the tunable waveguide device.

The embodiments described below are not limited to implementations that solve any or all disadvantages of known tunable waveguide devices.

Disclosure of Invention

This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the detailed description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.

A tunable element for an optical waveguide device such as an Optical Phased Array (OPA) is described. The tunable element comprises three waveguides arranged such that light propagates through a first waveguide, then through a second waveguide, then through a third waveguide, where the light is evanescently or directly coupled from one waveguide to the next. The tunable element further includes one or more resistive heating pads formed proximate to the second waveguide portion. The first waveguide and the third waveguide are formed of a first material, and the second waveguide is formed of a second, different material, and the second material is more thermo-optically sensitive than the first material.

A first aspect provides a tunable element for an optical waveguide arrangement, the tunable element comprising: a first waveguide part formed of a first material; a second waveguide formed of a second material and arranged to receive light coupled from the first waveguide; a third waveguide formed of the first material and arranged to receive light coupled from the second waveguide; and one or more resistive heating pads proximate to the second waveguide portion, wherein the second material is more thermo-optically sensitive than the first material.

A second aspect provides a tunable optical waveguide device comprising one or more tunable elements as described herein.

A third aspect provides an optical phased array comprising one or more tunable elements as described herein.

A fourth aspect provides a method of manufacturing a tunable element for an optical waveguide arrangement, the method comprising: forming a first waveguide and a third waveguide from a first material; forming a second waveguide portion from a second material; forming at least one layer of cladding material on the second waveguide portion; and forming one or more resistive heating pads on top of the cladding material layer and proximate to the second waveguide portion, wherein the second material is more thermo-optically sensitive than the first material.

As will be apparent to those skilled in the art, the preferred features may be combined as appropriate and with any of the aspects of the invention.

Drawings

Embodiments of the invention will be described, by way of example, with reference to the following drawings, in which:

FIGS. 1A and 1B show schematic diagrams of two example tunable waveguide devices;

FIGS. 2A, 2B and 2C show schematic diagrams of known tuning elements for tunable waveguide devices;

FIG. 3A is a schematic diagram of a tuning element for a first exemplary refinement of a tunable waveguide arrangement;

FIGS. 3B, 3C and 3D show cross-sections through three different variations of the improved tuning element shown in FIG. 3A;

figure 3E shows another cross-section through the improved tuning element shown in figure 3A;

FIG. 3F is a schematic diagram of a tuning element for a second exemplary refinement of a tunable waveguide arrangement;

FIG. 3G is a cross-section through the improved tuning element shown in FIG. 3F;

FIG. 4A is a schematic diagram of a tuning element for a third exemplary refinement of a tunable waveguide arrangement;

4B, 4C and 4D show cross-sections through three different variations of the improved tuning element shown in FIG. 4A;

figure 4E shows another cross-section through the improved tuning element shown in figure 4A;

FIG. 4F is a schematic diagram of a fourth exemplary improved tuning element for a tunable waveguide apparatus;

figure 4G is a cross-section through the improved tuning element shown in figure 4F;

FIG. 5 is a schematic diagram of an example tunable waveguide assembly including a plurality of improved tuning elements;

6A, 6B, 6C and 6D illustrate four different example methods of fabricating an improved tuning element for a tunable waveguide arrangement;

FIGS. 7A, 7B, 7C and 7D illustrate four different exemplary methods of fabricating an improved tuning element for a tunable waveguide arrangement; and

fig. 8A, 8B, 8C, and 8D illustrate four different example methods of fabricating an improved tuning element for a tunable waveguide arrangement.

Common reference numerals are used throughout the figures to indicate similar features.

Detailed Description

Embodiments of the present invention are described below by way of example only. These embodiments represent the best modes presently known to the applicant for carrying out the invention, but they are not the only modes in which this can be achieved. The description sets forth the functions of the example and the sequence of steps for constructing and operating the example. However, the same or equivalent functions and sequences may be accomplished by different examples.

As mentioned above, tunable waveguide devices, such as OPAs or VOAs (variable optical attenuators), may be fabricated, for example, using the thermo-optic effect. Such tunable waveguide devices operate by splitting an optical path 102 into multiple optical paths 104, 106, each of which includes a separate waveguide, introducing a variable phase change into one or more optical paths (using one or more tuning elements 112 that rely on thermo-optic effects), and then recombining light from some or all of the multiple optical paths. Any phase change introduced by one or more tuning elements 112 causes the light to recombine differently (e.g., as a result of interference between light from different optical paths). An exemplary tunable waveguide apparatus 100 is shown in fig. 1A.

The tuning element 112 is capable of changing the phase of the output light and the phase change is achieved by heating the waveguide to change the optical properties (i.e., refractive index) of the waveguide (i.e., core) material within the tuning element. Heating may be achieved by passing an electrical current through a resistive heating pad (which may be formed, for example, from a metal, doped silicon strip, or other material) that is positioned proximate to the waveguide (e.g., above or beside the waveguide).

Although fig. 1A shows an example tunable waveguide apparatus that uses two different splitter structures 108, 110 to split and then recombine light, in other examples, such as the second example tunable waveguide apparatus 120 shown in fig. 1B, the waveguides forming each of the multiple optical paths may be terminated with a reflective element 114 such that the same splitter structure 108 is used to split and recombine light. Although the two example apparatus 100, 120 shown in fig. 1A and 1B show light being split into two optical paths, in other examples, light may be split into more than two optical paths (e.g., into eight optical paths) using any suitable splitter arrangement.

In the example apparatus 100, 120 shown in fig. 1A and 1B, there is a single tuning element 112 in one of the multiple optical paths 104, 106; however, in other examples, there may be a tuning element in more than one of the plurality of optical paths, and in various examples, there may be a tuning element in each of the plurality of optical paths. In examples where the tunable element is included in more than one of the plurality of optical paths, the tunable elements within different optical paths may introduce different phase changes (e.g., by different sizes of the tunable element and/or passing different currents through resistive heating pads within the tunable element). Various techniques, such as trenches formed in the cladding material, may be used to provide thermal isolation between adjacent tunable elements.

Fig. 2A shows a schematic diagram (plan view) of a tunable element (e.g., tunable element 112) in a waveguide apparatus. The tunable element includes a waveguide (i.e., waveguide core) 202 and a resistive heating pad 204. The direction of propagation of the light is illustrated by the arrows in fig. 2A, although as described above (e.g., with reference to fig. 1B), in various examples, the light may also propagate through the tunable element in the opposite direction.

Fig. 2B and 2C show two example cross-sections through the tunable element of fig. 2A. The first cross-section shown in fig. 2B is taken perpendicular to the direction of propagation of light through the waveguide 202, as indicated by the dashed line Y-Y 'in fig. 2A, and the second cross-section shown in fig. 2C is taken through the center of the waveguide 202 and along the direction of propagation of light through the waveguide 202, as indicated by the dashed line X-X' in fig. 2A.

As shown in fig. 2B and 2C, the waveguide 202 is surrounded by cladding material 203. In fig. 2B and 2C, the resistive heating pad 204 is shown as being formed in a layer above the waveguide 202, and there is a thin layer of cladding material 203 between the waveguide 202 and the heating pad 204. However, in other examples, the resistive heating pad 204 may be formed in a layer below the waveguide 202, or may be formed in substantially the same plane as the waveguide 202 and adjacent to the waveguide 202. In many examples (e.g., where the heating pad 204 is formed of a metal), there is a layer of cladding material 203 between the waveguide 202 and the heating pad 204 (e.g., to prevent absorption of the metal into the waveguide 202), and the thickness of this layer may be, for example, in the range of less than one micron to several microns. For example, the thickness of the layer may be selected based on optical loss targets and thermal efficiency of the device.

Heating of the waveguide 202 by the resistive heating pad 204 changes the refractive index in the heated portion of the waveguide 202, and this change in refractive index results in a phase change of the light output from the tunable element. Any lack of control or precision in the heating of the waveguide 202 (e.g., any unpredictable or variable thermal diffusion in the waveguide 202) results in phase errors, and this can significantly impair the performance of the tunable waveguide apparatus that includes the tunable element.

An improved tunable element is described herein in which phase errors introduced by heating of the waveguide are reduced or eliminated. Such improved tunable elements may form part of any tunable waveguide device (e.g., OPA or VOA) and/or integrated photonic system. As described in detail below, the improved tunable element includes three adjacent waveguide sections along the propagation direction of light, such that light is coupled from the first section into the second section and from the second section into the third section. The first and third portions are formed of a first material (e.g., SiN or SiON) or combination of materials, and the second portion is formed of a second material (e.g., silicon, amorphous silicon, or polysilicon) or combination of materials that is more thermo-optically sensitive than the first material. The improved tunable element further comprises a resistive heating pad having the same length (measured along the direction of propagation of the light) as the length of the second portion. All three portions of the waveguide are surrounded by cladding material (e.g., silicon oxide or polymer).

The first and third portions may be referred to as "input", "output" or "input/output" portions of the waveguide, and the second portion may be referred to as a "central" portion of the waveguide. As described above, light travels through the first section, then through the second section, then through the third section (or in reverse order for light traveling in the opposite direction).

The term "thermo-optic sensitivity" is used herein to refer to the change in refractive index with temperature (denoted dn/dT, where dn is the change in refractive index due to a change in temperature dT), where higher thermo-optic sensitivity refers to materials with larger values of dn/dT.

By using a material with a higher thermo-optic sensitivity only for the waveguide portion (i.e. the second portion) close to the resistive heating pad, any phase change caused by unintentional heating of the other (less thermo-sensitive) waveguide material (i.e. the first and/or third portion) will be significantly smaller than the phase change caused by heating of the second portion. Such phase changes due to unintentional heating (e.g., heat dissipation beyond the second portion of the waveguide) affect the phase error, which is reduced as compared to tuning elements where the entire waveguide (i.e., the waveguide core) is formed of the same material (e.g., as shown in fig. 2A-2C).

In addition, by using a material with a higher thermo-optic sensitivity for the waveguide portion (i.e., the second portion) near the resistive heating pad, less energy is required to produce the desired phase change in the propagating light, and thus the entire tunable element 300 is more efficient than a tunable device having a waveguide formed entirely of the first material.

In addition, by using only the material with the higher thermo-optic sensitivity for the portion of the waveguide near the resistive heating pad, i.e., the second portion, and not for the remaining portions of the waveguide, i.e., the first and third portions, the overall optical loss of the system can be reduced. For example, in the case where Si is used for the second portion, the waveguide loss may be 1.5dB/cm, while the waveguide loss of SiN (which may be used for the first and third portions) may be less than 0.5 dB/cm.

Figure 3A shows a schematic diagram (plan view) of an exemplary improved tunable element 300. The improved tunable element may be incorporated, for example, in a waveguide apparatus (e.g., tunable element 112) such as that shown in fig. 1A and 1B. The improved tunable element 300 includes three waveguide (i.e., waveguide core) portions 301-303 and a resistive heating pad 304 (which may be formed of TiN, for example). The direction of propagation of the light is illustrated by the arrows in fig. 3A, although as described above (e.g., with reference to fig. 1B), in various examples, the light may also propagate through the tunable element 300 in the opposite direction.

As described above, the first waveguide part 301 and the third waveguide part 303 are formed of a first material having a first dn/dT value S1, and the second (or center) waveguide part 302 is formed of a second material having a second dn/dT value S2. The second material is more thermo-optically sensitive than the first material, i.e. S2> S1.

In an example, the first material is silicon nitride SiN, and S1 ═ 2.4 × 10^-5And the second material is silicon Si, and S2 ═ 1.8 × 10^-4. In this example, the thermo-optic sensitivity of the second material is an order of magnitude greater than the thermo-optic sensitivity of the first material. Thus, any change in refractive index in the first and/or third portions of the waveguide caused by heating from the resistive heating pad will result in a very small (e.g., negligible) phase change (and thus a very small phase error) compared to the phase change caused by heating of the second waveguide portion.

Silicon and silicon nitride provide only one example combination of materials that may be used to form the three waveguide portions 301-303. In other examples, polysilicon or amorphous silicon may be used for the second waveguide section 302. In another example, the first material is SiON and the second material is silicon, polysilicon, or amorphous silicon. In another example, the first material is SiON and the second material is SiN. In further examples, any combination of materials may be used, where the second material has a higher thermo-optic sensitivity than the first material. In other examples, the first and third portions may not be made of a single material, but may be formed of a first material combination (e.g., a multilayer structure forming a waveguide core). Similarly, the second portion may not be made of a single material, but may be formed of a combination of second materials (e.g., forming a second multilayer structure of the waveguide core that is different from the structures for the first and third portions).

The first waveguide 301 and the third waveguide 303 may be formed in the same layer and thus may be coplanar. The second waveguide portion 302 may be formed in a separate process step (because it is formed of a different material) and may be formed in a layer separate from the first and third waveguide portions, as shown in fig. 3B and 3C. The manufacturing process is described below with reference to fig. 6A-6D.

Figures 3B, 3C and 3D show three different alternative cross-sections through the tunable element 300 of figure 3A, and in all these figures the cross-sections are taken through the center of the waveguide 301-303 and along the propagation direction of light through the waveguide 301-303, as indicated by the dashed line X-X' in figure 3A. In all examples, the waveguide portions 301-303 are surrounded by the cladding material 203, and in all examples shown, the length of the second waveguide portion 302 (measured along the propagation direction of the light) is the same as the length of the resistive heating pad 304, and the end of the second waveguide portion 302 and the end of the resistive heating pad 304 are aligned (in a direction parallel to the propagation direction of the light). However, in other examples (not shown in the figures), the length of the second waveguide 302 (measured along the direction of propagation of the light) may be longer (e.g., only slightly longer) than the length of the resistive heating pad 304, such that the resistive heating pad 304 terminates before the end of the second waveguide 302. In the first two examples, as shown in fig. 3B and 3C, a multilayer waveguide structure is formed in which the second waveguide 302 is in a plane (or layer) parallel to but spaced apart from the planes (or layers) of the first and third waveguides 301 and 303. Light is vertically coupled from the end of the first waveguide 301 and enters the second waveguide 302 (i.e., enters the proximal end of the second waveguide 302), and then is vertically coupled from the other end of the second waveguide 302 and enters the third waveguide 303 (i.e., enters the proximal end of the third waveguide 303).

To improve the efficiency of vertical coupling, the waveguides may overlap by a small amount (i.e., the first waveguide and the second waveguide overlap by a small amount, and the second waveguide and the third waveguide overlap by a small amount), as shown in fig. 3B and 3C. This means that the resistive heating pad 304 may overlap the ends of the first and third waveguides; however, any phase error introduced by such overlap is very small due to the reduced thermal sensitivity of the materials forming the first and third waveguides.

In the first example shown in fig. 3B, the plane of the second waveguide 302 is further away from the resistive heating pad 304 than the plane of the first waveguide 301 and the third waveguide 303, i.e., in the orientation shown in fig. 3B, the plane of the first waveguide 301 and the third waveguide 303 is above the plane of the second waveguide 302 and below the plane of the resistive heating pad 304. In the second example shown in fig. 3C, the plane of the second waveguide 302 is closer to the resistive heating pad 304 than the plane of the first and third waveguides 301, 303, i.e. in the orientation shown in fig. 3C, the plane of the first and third waveguides 301, 303 is below the plane of the second waveguide 302, and this plane is in turn below the plane of the resistive heating pad 304.

In the third exemplary cross-section shown in fig. 3D, there is no multilayer waveguide structure, but all three waveguide sections 301-303 are substantially in the same plane, wherein light is butt-coupled between the first and second sections 301, 302 and between the second and third sections 302, 303. Depending on the way vertical coupling (in the examples of fig. 3B and 3C) is achieved, butt coupling may be more lossy, so the multilayer structure of fig. 3B or 3C may result in a tunable element with less optical loss.

FIG. 3E illustrates another cross-section through the tunable element 300 shown in FIG. 3A; however, unlike the previous cross-sections (shown in fig. 3B-3D), the cross-section shown in fig. 3E is taken perpendicular to the direction of propagation of light through the second portion 302 of the waveguide, as shown by the dashed line Y-Y' in fig. 3A. As shown in fig. 3E, although the resistive heating pad 304 has the same length as the second portion 302 of the waveguide (as measured along the direction of propagation), the resistive heating pad 304 may be wider than the second portion 302 of the waveguide (as measured perpendicular to the direction of propagation), while in other examples the resistive heating pad 304 and the second portion 302 of the waveguide may have the same width or the resistive heating pad 304 may be narrower than the second portion 302 of the waveguide.

Although fig. 3A-3E show that the dimensions of the waveguides (e.g., width and thickness, where the width is measured perpendicular to and in the plane of the waveguide, and the thickness is measured perpendicular to both the propagation direction and the plane of the waveguide) are substantially the same, in most examples, the width and/or thickness of the second waveguide 302 is different than the width and/or thickness of the first and third waveguides 301, 303, where the first and third waveguides 301, 303 have the same width and thickness. Although the actual dimensions of the different waveguides may be chosen when designing the tuning element 300 (or the waveguide arrangement comprising the tuning element 300), if the refractive index of the second material (forming the second waveguide 302) is higher than the refractive index of the first material, the propagating light is more strongly confined within the waveguide, and the width and/or thickness of the second waveguide 302 may be smaller than the width and/or thickness of the first and third waveguides 301, 303. In one example, the first waveguide 301 and the third waveguide 303 may be formed of SiN (n ═ 2) and may be 400nm thick and 1000nm wide, while the second waveguide 302 may be formed of Si (n ═ 3.5) and may be 220nm thick and 500nm wide.

Furthermore, although FIGS. 3B-3E illustrate the resistive heating pad 304 formed above all of the waveguides 301-303, in other examples, the orientation may be reversed such that the resistive heating pad 304 may be formed in a plane below all of the waveguides 301-303.

Figures 3F and 3G illustrate variations of the tunable element 300 shown in figures 3A-3E and described above. In this example tunable element 320, instead of a single resistive heating pad 304 in a layer above or below the second waveguide section 302, there are two resistive heating pads 304A, 304B, one on either side of the second waveguide section 402, and in substantially the same plane (or layer) as the second waveguide section 402. FIG. 3F shows a plan view and a cross-section (as indicated by the dashed line X-X' in FIG. 3F) along the propagation direction of light through the waveguide portions 301-303 may be as shown in any of FIGS. 3B-3D, omitting the resistive heating pad 304. Figure 3G shows another cross-section through the tunable element 320 shown in figure 3F, taken perpendicular to the direction of propagation of light through the second portion 302 of the waveguide, as shown by the dashed line Y-Y' in figure 3F. The cross-section shows two resistive heating pads 304A, 304B formed in the same plane as the second waveguide section 302 and located on either side of the second waveguide section 202. The resistive heating pads 304A, 304B may be formed of any suitable material (e.g., TiN), and may be the same thickness as the central waveguide section 302, or thicker or thinner than the central waveguide section 302.

Figure 4A shows a schematic diagram (plan view) of another exemplary improved tunable element 400. The tunable element 400 is a variation of the tunable element described above with reference to fig. 3A-3E, and is described in more detail below. In this modification, as shown in fig. 4A, the first waveguide 301 and the third waveguide 303 are as described above with reference to fig. 3A to 3E; however, the second waveguide portion 402 is different in cross section from the second waveguide portion 302 described above. The second waveguide part 402 in this variant is a rib waveguide, thus having a thicker part 402A in the centre of the waveguide than the two outer parts 402B, 402C, and this is also shown in fig. 4E (described below). The thickness of the central portion 402A may be substantially the same thickness as the second waveguide portion 302 in the previous example. Furthermore, the resistive heating pad arrangement in the tunable element 400 is different from the resistive heating pad 304 in the tunable element 300. In particular, instead of a single resistive heating pad 304 in a layer above or below the second waveguide portion 302, there are two resistive heating pads 404A, 404B on either side of the second waveguide portion 402 and substantially in the same plane (or layer) as the second waveguide portion 402, and these may be formed, for example, of doped silicon, rather than of one or more metals (e.g., TiN). Any suitable dopant (e.g. p-type or n-type) may be used and the thermal efficiency will depend on the resistance of the heating pad and hence the doping concentration. The direction of propagation of the light is illustrated by the arrows in fig. 4A, although as described above (e.g., with reference to fig. 1B), in various examples, the light may also propagate through the tunable element 400 in the opposite direction.

The two resistive heating pads 404A, 404B may be referred to as resistive regions of the second material. These are formed by implanting impurities into the region (i.e. doping) and this changes the local resistivity of the part of the structure formed of the second material. As described above, the first and third waveguides 301, 303 are formed of a first material having a first dn/dT value S1, and the second (or center) waveguide 402 is formed of a second material having a second dn/dT value S2. The second material is more thermo-optically sensitive than the first material, i.e. S2> S1.

The first waveguide 301 and the third waveguide 303 may be formed in the same layer and thus may be coplanar. The second waveguide portion 402 may be formed in a separate process step (because it is formed of a different material), and may be formed in a layer separate from the first and third waveguide portions, as shown in fig. 4B and 4C. The manufacturing process is described below with reference to fig. 7A-7D.

Fig. 4B, 4C and 4D show three different alternative cross-sections through the tunable element 400 of fig. 4A, and in all these figures the cross-sections are taken through the center of the waveguides 301, 402, 303 and along the propagation direction of light through the waveguides 301, 402, 303, as indicated by the dashed line X-X' in fig. 4A. In all examples, the waveguides 301, 402, 303 are surrounded by the cladding material 203, and in all examples shown, the length of the second waveguide 402 (measured along the propagation direction of the light) is the same as the length of the resistive heating pads 404A, 404B, and the ends of the second waveguide 402 and the resistive heating pads 404A, 404B are aligned (in a direction parallel to the propagation direction of the light). However, in other examples (not shown in the figures), the length of the second waveguide 402 (as measured along the direction of propagation of the light) may be longer (e.g., only slightly longer) than the length of the resistive heating pads 404A, 404B, such that the resistive heating pads 404A, 404B terminate before the ends of the second waveguide 402.

In the first two examples, as shown in fig. 4B and 4C, a multilayer waveguide structure is formed in which the second waveguide 402 is in a plane (or layer) parallel to but spaced apart from the planes (or layers) of the first and third waveguides 301 and 303. Light is vertically coupled from the end of the first waveguide 301 and enters the second waveguide 402 (i.e., enters the proximal end of the second waveguide 402), and then is vertically coupled from the other end of the second waveguide 402 and enters the third waveguide 303 (i.e., enters the proximal end of the third waveguide 303). To improve the efficiency of vertical coupling, the waveguides may overlap by a small amount (i.e., the first waveguide and the second waveguide overlap by a small amount, and the second waveguide and the third waveguide overlap by a small amount), as shown in fig. 4B and 4C. In the first example shown in fig. 4B, the planes of the first waveguide 301 and the third waveguide 303 are above the plane of the second waveguide 402, and in the second example shown in fig. 4C, the planes of the first waveguide 301 and the third waveguide 303 are below the plane of the second waveguide 402.

In a third example cross-section shown in fig. 4D, there is no multilayer waveguide structure, but all three waveguide parts 301, 402, 303 are substantially in the same plane, wherein light is butt-coupled between the first and second parts 301, 402 and between the second and third parts 402, 303. As described above, depending on the way vertical coupling (in the examples of fig. 4B and 4C) is achieved, butt coupling may be more lossy, and thus the multilayer structure of fig. 4B or 4C may result in a tunable element with less optical loss.

FIG. 4E illustrates another cross-section through tunable element 400 shown in FIG. 4A; however, unlike the previous cross-sections (shown in FIGS. 4B-4D), the cross-section shown in FIG. 4E is taken perpendicular to the direction of propagation of light through the second portion 402 of the waveguide, as shown by the dashed line Y-Y' in FIG. 4A. The cross-section shows the difference in thickness between the central portion 402A and the outer portions 402B, 402C of the second waveguide part. As shown in fig. 4E, the resistive heating pads 404A, 404B are formed in the same plane as the second waveguide section 402 and are located on either side of the second waveguide section 402 and adjacent to the outer portions 402B, 402C of the second waveguide section 402. As described below, the resistive heating pads 404A, 404B may be initially formed (e.g., deposited) in the same process step as the second waveguide 402 and from the same material as the second waveguide 402, followed by another process step for doping (i.e., selectively doping) the portion of the material that will form the resistive heating pads 404A, 404B.

A variation of the tunable element 400 shown in fig. 4A-4E and described above is shown in fig. 4F and 4G. In this example tunable component 420, instead of resistive heating pads 424A, 424B formed of doped silicon (as described above), they are formed of another material deposited onto the outer portions 402B, 402B of the second waveguide section 402. Fig. 4F shows that a plan view and a cross-section (as indicated by the dashed line X-X' in fig. 4F) along the propagation direction of light through the waveguide portions 301, 402, 303 may be as shown in any of fig. 4B-4D and described above. Figure 4G shows another cross-section through the tunable element 420 shown in figure 4F, taken perpendicular to the direction of propagation of light through the second portion 402 of the waveguide, as shown by the dashed line Y-Y' in figure 4F. The cross-section shows two resistive heating pads 424A, 424B deposited on distal portions of the outer portions 402B, 402B of the second waveguide portion 402. The resistive heating pads 424A, 424B may be formed of any suitable material (e.g., TiN) and may be the same thickness as the resistive heating pads 304 described above with reference to fig. 3A-3G, or thicker or thinner than the resistive heating pads 304. While fig. 4B-4C show that the dimensions (e.g., width and thickness, with the width measured perpendicular to and in the plane of the waveguide, and the thickness measured perpendicular to both the propagation direction and the plane of the waveguide) of the thicker portion of the second waveguide 402A are substantially the same, in most examples, the width and/or thickness of the thicker portion 402A of the second waveguide is different than the width and/or thickness of the first waveguide 301 and the third waveguide 303, with the first waveguide 301 and the third waveguide 303 having the same width and thickness. Although the actual dimensions of the different waveguides may be chosen when designing the tuning element 400 (or the waveguide arrangement comprising the tuning element 400), if the refractive index of the second material (which forms the second waveguide 402) is higher than the refractive index of the first material, the propagating light is more strongly confined within the waveguide, and the width and/or thickness of the thicker portion 402A of the second waveguide may be smaller than the width and/or thickness of the first and third waveguides 301, 303. In one example, the first and third waveguides 301, 303 may be formed of SiN (n 2) and may be 400nm thick and 1000nm wide, while the second waveguide 402 may be formed of Si (n 3.5), and the thicker portion 402A may be 220nm thick and 500nm wide, while the outer portions 402B, 402C and resistive heating pads 404A, 404B may be significantly thinner (e.g., tens of nanometers).

In all of the above examples, the first waveguide part 301 and the third waveguide part 303 are formed of a first material having a first value of dn/dT S1, and the second (or center) waveguide part 302 is formed of a second material having a second value of dn/dT S2. The second material is more thermo-optically sensitive than the first material, i.e. S2> S1. In a variation of any of the examples above, the first and third waveguides may be formed of different materials, e.g., the first waveguide may be formed of a first material having a first value of dn/dT S1, and the third waveguide may be formed of a third material having a third value of dn/dT S3, wherein the second material (forming the second waveguide) is more thermo-optically sensitive than both the first and third materials, i.e., S2> S1 and S2> S3.

The improved tunable elements 300, 320, 400, 420 described above with reference to fig. 3A-3G and 4A-4G may be incorporated into (i.e., form part of) any tunable waveguide device, and two examples 100, 120 are shown in fig. 1A and 1B. Another example tunable waveguide apparatus 500 is shown in fig. 5. The tunable waveguide arrangement 500 shown in fig. 5 is an OPA (optical phased array) and comprises a single input waveguide (shown on the left side of fig. 5), a plurality of tunable elements 501 (eight in the example shown in fig. 5) and a plurality of output waveguides (shown on the right side of fig. 5). The tunable element 501 shown in fig. 5 may be as described above with reference to any of fig. 3A-3G and fig. 4A-4G, or may be any combination of the tunable elements (or portions thereof) described above. As shown in fig. 5, in a tunable waveguide apparatus 500, an arrangement of splitters 502 is formed that is configured to split incident light into a plurality of separate optical paths (eight in the example shown in fig. 5), and each of these separate optical paths includes a tunable element 501 (e.g., tunable elements 300, 320, 400, 420). In the example shown in fig. 5, the tunable element 501 in each path has a resistive heating structure 504 (and thus also the second waveguide section 502) of different length, but the width of the resistive heating structure 504 (e.g., one or more resistive heating pads as described above) may be substantially the same. Thermal isolation trenches 506 are formed between each tunable element 500 to reduce diffusion from one resistive heating pad 504 and cause any heating of the adjacent tunable elements or refractive index changes in the adjacent optical paths. The second waveguide section 502 in the tunable element 500 may be the second waveguide section 302 described above with reference to fig. 3A-3G or the second waveguide section 402 described above with reference to fig. 4A-4G. Similarly, the resistive heating structure 504 in the tunable element 500 may be the resistive heating pad 304 described above with reference to fig. 3A-3E or two resistive heating pads 304A, 304B, 404A, 404B described above with reference to any of fig. 3F-3G and 4A-4E.

It should be understood that the figures in fig. 3A-3G, 4A-G, and 5 do not necessarily show the entire structure or material stack, and that additional layers or elements not shown in the figures may be present (e.g., additional metal layers to provide traces that are electrically connected to the resistive heating pads).

Fig. 6A-6D illustrate four different example methods of fabricating an improved tunable element 300 as described herein. Figures 7A-7D illustrate four different example methods of fabricating an improved tunable element 400 as described herein. Figures 8A-8D illustrate four different example methods of fabricating improved tunable elements 320, 420 as described herein. While these methods are described in isolation from the rest of the tunable waveguide arrangement, it should be understood that in many examples, the improved tunable elements 300, 320, 400, 420 are fabricated simultaneously with the rest of the tunable waveguide arrangement.

In the example methods shown in fig. 6A-6D, 7A-7D, and 8A-8D, there are many common stages, and in all methods, the stages performed are substantially the same (some cladding layers are omitted in some examples); however, they may be performed in a different order in order to produce the different layer structures shown in FIGS. 3B-3D, 3G, 4B-4D, and 4G. There may also be additional manufacturing stages not shown in fig. 6A-6D, 7A-7D, and 8A-8D, such as deposition of additional metal layers, cleaning stages, and the like.

Figure 6A illustrates an example method of fabricating an improved tunable element having a cross-section as shown in figure 3B. A first cladding layer is formed (block 602) and this may be formed directly on a substrate (e.g., a silicon wafer) or a partially processed substrate (e.g., such that the cladding is not formed directly on the surface of the substrate, but is formed on a material that has been deposited on the substrate). In various examples, the first cladding layer may include a buried oxide layer. Then, prior to depositing the second cladding layer (block 606), the second waveguide portion 302 may be formed of a second material on top of the first cladding layer (block 604). Then, prior to depositing the third cladding layer (block 608), the first waveguide 301 and the third waveguide 303 may be simultaneously formed on top of the second cladding layer (block 608). The resistive heating pad 304 is then deposited on top of the third cladding layer (block 612) and then covered by the fourth cladding layer (block 614).

Figure 6B illustrates an example method of fabricating an improved tunable element having a cross-section as shown in figure 3C. The method is similar to that shown in fig. 6A and described above (e.g., similar in forming a multilayer waveguide structure); however, as shown in fig. 6B, prior to depositing the second cladding layer (block 606), the first waveguide 301 and the third waveguide 303 are simultaneously formed on top of the first cladding layer (block 608). Then, the second waveguide 302 is formed on top of the second cladding layer (block 604) before depositing the third cladding layer (block 608). The resistive heating pad 304 is then deposited on top of the third cladding layer (block 612) and then covered by the fourth cladding layer (block 614), as previously described.

Fig. 6C and 6D illustrate two different example methods of fabricating an improved tunable element having the cross-section shown in fig. 3D. The method of FIGS. 6C and 6D is similar to the method shown in FIGS. 6A and 6B; however, since the first, second and third waveguides 301-303 are substantially coplanar, one cladding layer deposition step is eliminated (i.e., block 614 is omitted), and the second waveguide 302 may be formed directly after forming the first and third waveguides 301, 303 (in block 608) (as in FIG. 6C) or directly before forming the first and third waveguides 301, 303 (as in FIG. 6D) (in block 604). In the method of fig. 6C, a continuous waveguide core may be initially formed from a first material (in block 608) such that the first and third waveguides are connected, and then (also in block 608) a portion of the waveguide formed from the first material may be removed to leave a gap where a second material may be deposited (in block 604) to form a second waveguide.

The methods shown in fig. 7A-7D are very similar to those shown in fig. 6A-6D and described above. As shown, instead of forming the second waveguide portion (in block 604) and the resistive heating pad (in block 612) in separate processing stages, the formation of the link structure (the second waveguide portion 402 and the resistive heating pads 404A, 404B) is now performed. Initially, undoped regions that ultimately form the resistive heating pad are formed concurrently with the second waveguide portions (block 704), and these regions are then doped (block 705). The undoped region formed of the second material concurrently with the second waveguide portion (in block 704) has dimensions of a resistive heating pad, and the doping changes thermal and electrical properties of the region. Although not shown in fig. 7A-7D, it should be understood that the formation of the second waveguide portion 402 (in block 704) may involve an etching process or two separate deposition stages in order to form the stepped cross-section as shown in fig. 4E.

The methods shown in fig. 8A-8D are very similar to those shown in fig. 6A-6D and described above. As shown in the figure, instead of forming a single resistive pad on top of the cladding layer, the resistive pad is formed directly on the second waveguide portion (in block 812). Although not shown in fig. 8A-8D, it should be understood that the formation of the second waveguide portion 402 (in block 804) may involve an etching process or two separate deposition stages in order to form the stepped cross-section as shown in fig. 4G.

The techniques for forming the cladding layer and the waveguide may be different depending on the materials used to form them. For example, in the case where the second waveguide is formed of crystalline silicon, it cannot be formed by deposition, and thus a cladding layer (e.g., a first cladding layer) below the second waveguide may be a Buried Oxide (BOX) layer. Furthermore, because crystalline silicon cannot be formed by deposition, crystalline silicon cannot be used to form the second waveguide portion in the variation of the improved tunable element shown in fig. 3C and 4C. If silicon is used to form the second waveguide portion in these variations, polycrystalline or amorphous silicon may be used.

A first further example provides a tunable element for an optical waveguide arrangement, the tunable element comprising: a first waveguide part formed of a first material; a second waveguide formed of a second material and arranged to receive light coupled from the first waveguide; a third waveguide formed of the first material and arranged to receive light coupled from the second waveguide; and one or more resistive heating pads proximate to the second waveguide portion, wherein the second material is more thermo-optically sensitive than the first material.

The first waveguide and the third waveguide may be formed in a first layer, and the second waveguide may be formed in a second layer.

In use, light may be vertically coupled between one end of the first waveguide and the proximal end of the second waveguide, and may also be vertically coupled between the opposite end of the second waveguide and the proximal end of the third waveguide.

The tunable element may also include a cladding material layer between the first layer and the second layer.

The one or more resistive heating pads may be formed in the third layer. The tunable element may further include a cladding material layer between the second layer and the third layer.

The tunable element may further include a cladding layer between the first layer and the third layer such that the second layer is further from the third layer than the first layer.

The one or more resistive heating pads may be formed in the second layer. The one or more resistive heating pads may be formed from a resistive region of a second material.

The second waveguide portion may include a central portion and two outer portions, wherein the central portion is thicker than the outer portions.

The first waveguide, the second waveguide, and the third waveguide may be formed in a single layer.

In use, light may be butt-coupled between one end of the first waveguide and the proximal end of the second waveguide, and may also be butt-coupled between the opposite end of the second waveguide and the proximal end of the third waveguide.

The resistive heating pad may be formed in the second layer.

The tunable element may also include a cladding material layer between the first layer and the second layer.

The second waveguide may include a central portion and two outer portions, wherein the central portion is thicker than the outer portions, and wherein the resistive heating pad may be formed on top of each of the two outer portions of the second waveguide.

The first material may be silicon nitride and the second material may be silicon, amorphous silicon or polysilicon.

Each resistive heating pad may have a length that is the same as the length of the second waveguide such that each resistive heating pad is proximate to the second waveguide and not proximate to the first or third waveguides.

A second further example provides a tunable optical waveguide apparatus that includes the one or more tunable elements of the first further example.

A third further example provides an optical phased array that includes the one or more tunable elements of the first further example.

A fourth additional example provides a method of manufacturing a tunable element for an optical waveguide arrangement, the method comprising: forming a first waveguide and a third waveguide from a first material; forming a second waveguide portion from a second material; forming at least one layer of cladding material on the second waveguide portion; and forming one or more resistive heating pads on top of the cladding material layer and proximate to the second waveguide portion, wherein the second material is more thermo-optically sensitive than the first material.

As will be apparent to those skilled in the art, any of the ranges or device values given herein may be extended or altered without losing the effect sought.

It should be understood that the benefits and advantages described above may relate to one embodiment or may relate to several embodiments. Embodiments are not limited to those embodiments that solve any or all of the problems or those embodiments having any or all of the benefits and advantages described.

Any reference to "an" item refers to one or more of those items. The term "comprising" is used herein to mean including the identified method blocks or elements, but that such blocks or elements do not include an exclusive list, and that the method or apparatus may contain additional blocks or elements.

The steps of the methods described herein may be performed in any suitable order, or simultaneously where appropriate. Additionally, individual blocks may be deleted from any of the methods without departing from the spirit and scope of the subject matter described herein. Aspects of any of the examples described above may be combined with aspects of any other examples described to form further examples without losing the effect sought.

It should be understood that the above description of the preferred embodiments is given by way of example only and that various modifications may be made by those skilled in the art. Although various embodiments have been described above with a certain degree of particularity, or with reference to one or more individual embodiments, those skilled in the art could make numerous alterations to the disclosed embodiments without departing from the spirit or scope of this invention.

Claims (20)

1. A tunable element (300, 320, 400, 420, 501) for an optical waveguide arrangement, the tunable element comprising:

a first waveguide (301), the first waveguide (301) being formed of a first material;

a second waveguide (302, 402), the second waveguide (302, 402) being formed of a second material and arranged to receive light coupled from the first waveguide;

a third waveguide (303), the third waveguide (303) being formed of the first material and arranged to receive light coupled from the second waveguide; and

one or more resistive heating pads (304, 404A, 404B, 424A, 424B), the one or more resistive heating pads (304, 404A, 404B, 424A, 424B) proximate to the second waveguide portion,

wherein the second material is more thermo-optically sensitive than the first material.

2. The tunable element of claim 1, wherein the first and third waveguides (301, 303) are formed in a first layer and the second waveguide (302, 402) is formed in a second layer.

3. The tunable element of claim 2, wherein, in use, light is vertically coupled between one end of the first waveguide and the proximal end of the second waveguide, and is also vertically coupled between the opposite end of the second waveguide and the proximal end of the third waveguide.

4. The tunable element of claim 2 or 3, further comprising a cladding material layer between the first layer and the second layer.

5. The tunable element of any one of claims 2-4, wherein the one or more resistive heating pads (304) are formed in a third layer.

6. The tunable element of claim 5, further comprising a cladding material layer between the second layer and the third layer.

7. The tunable element of claim 5, further comprising a cladding layer between the first layer and the third layer such that the second layer is further from the third layer than the first layer.

8. The tunable element of any one of claims 2-4, wherein the one or more resistive heating pads (304A, 304B, 404A, 404B) are formed in the second layer.

9. The tunable element of claim 8, wherein the one or more resistive heating pads (404A, 404B) are formed by resistive regions of the second material.

10. The tunable element of claim 8 or 9, wherein the second waveguide part comprises a central portion (402A) and two outer portions (402B, 402C), wherein the central portion is thicker than the outer portions.

11. The tunable element of claim 1, wherein the first, second and third waveguides (301-303) are formed in a single layer.

12. The tunable element of claim 11, wherein, in use, light is butt-coupled between one end of the first waveguide and a proximal end of the second waveguide, and is also butt-coupled between an opposite end of the second waveguide and a proximal end of the third waveguide.

13. The tunable element of claim 11 or 12, wherein the resistive heating pad is formed in the second layer.

14. The tunable element of claim 13, further comprising a cladding material layer between the first layer and the second layer.

15. The tunable element of any one of claims 1-5, 11, and 12, wherein the second waveguide comprises a central portion (402A) and two outer portions (402B, 402C), wherein the central portion is thicker than the outer portions, and wherein the resistive heating pads are formed on top of each of the two outer portions of the second waveguide.

16. The tunable element of any one of the preceding claims, wherein the first material is silicon nitride and the second material is silicon, amorphous silicon or polysilicon.

17. The tunable element of any one of the preceding claims, wherein the length of each resistive heating pad (304) is the same as the length of the second waveguide (302), such that each resistive heating pad is close to the second waveguide and not to the first or third waveguide (301, 303).

18. A tunable optical waveguide arrangement comprising one or more tunable elements according to any one of the preceding claims.

19. An optical phased array comprising one or more tunable elements according to any of claims 1-17.

20. A method of manufacturing a tunable element for an optical waveguide device, the method comprising:

forming (608) the first and third waveguides from a first material;

forming (604, 704) a second waveguide portion from a second material;

forming (606, 610) at least one layer of cladding material on the second waveguide portion; and

forming (612,704) one or more resistive heating pads on top of the cladding material layer and proximate to the second waveguide portion,

wherein the second material is more thermo-optically sensitive than the first material.